This invention relates to a method for producing lithographic printing plates for plate-making by silver complex diffusion transfer process, which involves the scanning exposure technique to achieve high intensity short time exposure. It also relates to plate-making and printing methods using such lithographic printing plates.
Lithographic printing plates according to which the transfer silver images obtained by silver complex diffusion transfer process can be directly utilized as ink receptive areas are well known, for example, in Japanese Patent Publication (Kokoku) No. 30562/73, Japanese Patent Laid-Open Application (Kokai) Nos. 21602/78, 103104/79 and 9750/81.
According to a typical silver complex diffusion transfer process suitable for plate-making with such lithographic printing plates, a photosensitive material comprising a support and a subbing layer which also serves as antihalation layer, a silver halide emulsion layer and a physical development nuclei layer which are provided on the support is subjected to imagewise exposure and development treatment to convert silver halide of latent image areas to black silver in the emulsion layer. Simultaneously, silver halide of other than latent image areas dissolves with a silver halide complexing agent contained in the development processing solution and diffuses to the surface of the photosensitive material. The silver complex thus dissolved and diffused is precipitated on the physical development nuclei in the surface layer by the reducing action of developing agents. In order to increase the ink-receptivity of thus obtained silver images, if necessary, sensitizing treatment is conducted after the development treatment. Thereafter, the lithographic printing plate is set in an offset press and transfer of inked images to print materials is carried out.
According to the conventional methods, the silver halide emulsion layer is spectrally sensitized with merocyanine dyes, cyanine dyes and the like so that it has a sensitivity maximum at around 550 nm in the green area and this emulsion layer is subjected to exposure of several seconds to several ten seconds in a process camera having a normal light source such as tungsten light source.
A printing plate obtained in such a way, however, exhibited only limited sharpness and resolving power, even though an aforementioned lithographic printing plates inherently excellent in these properties has been used. Moreover, in reproducing a color print from a color original, the conventional process has disadvantages in that both procedures of preparing the lithographic printing plates and plate-making are troublesome in addition to the insufficient resolving power.
At present, for solving said problems there are considered direct plate-making methods by conducting a scanning type flash exposure at markedly high intensity and for a short period of time (10.sup.-5 seconds or less) with various laser beams such as neon-helium laser beam, light-emitting diodes (LED), cathode ray tubes (CRT). However, it is known that such exposing method causes the so-called high intensity reciprocity law failure which results in reduction of sensitivity and gradation. There are proposed many methods to prevent such reciprocity law failure. However, in the case of the sensitivity expressed by exposure required for forming ink-receptive transfer silver, only such sensitivity as utterly insufficient for the light sources such as LED of low output can be obtained and moreover printing characteristics are often lowered, e.g., to cause scumming. In order to provide high sensitivity to such exposing light sources as mentioned above, sensitizing dyes in conformity to the emission maximum wavelength of the light sources must be used.
Japanese Patent Laid-Open Application (Kokai) No. 21601/78 (U.S. Pat. No. 4,134,769) discloses use of betaine type or anion type cyanine sensitizing dyes to obtain offset printing plates of high sensitivity and improved in printabilities and it further shows that it is preferred for obtaining high sensitivity to use silver halide emulsions high in silver bromide content.
On the other hand, it is known that in the case of a high-intensity short-time exposure, silver halide emulsions high in silver chloride content are inferior in spectral sensitization effect to those low in silver chloride content (see, e.g., Japanese Patent Publication (Kokoku) No. 42172/73) and moreover the former is more susceptible to high-intensity reciprocity law failure than the latter (e.g., "Photographic Science and Engineering", Vol 26, 1982). Thus, generally silver halide emulsions high in silver chloride content can be said to be unsuitable as emulsions for scanning type high-intensity and short-period exposure.
As a result of the inventors' intensive researches on lithographic printing plates using silver complex diffusion transfer process which employs the light sources as mentioned hereinbefore, it has been found that emulsions higher in silver chloride content can provide lithographic printing plates higher in sensitivity and superior in printing characteristics.